Biocompatible polymer gels having low friction coefficients (high lubricity) when immersed in a liquid (such as bodily fluid for example) are known. However, such polymer gels often have unacceptable mechanical strength. Copolymerizing a hydrophilic monomer with a proper amount of a hydrophobic monomer in order to reduce the water content, which thereby increases the mechanical strength is known, however the friction coefficient of such polymer gels can increase and the lubricity can therefore decrease.
PCT Publication No. WO03/093337 discloses a method of forming an interpenetrating polymer network structure hydrogel or a semi-interpenetrating polymer network structure hydrogel by polymerizing and crosslinking a first monomer to form a network structure, introducing a second monomer into the network structure, polymerizing the second monomer and crosslinking (if necessary) the resulting polymer. This method improves the mechanical strength but the hydrogel lacks sufficient lubricity.
Japanese Unexamined Patent Publication (Kokai) No. 2002-212452 discloses a method to form a low friction hydrogel by mixing a linear polymer and a polymer gel, or graft-polymerizing a linear polymer to a surface of a polymer gel. However such a method may decrease the lubricity because graft chains or linear polymers may be cut due to friction or abrasion.
Japanese Unexamined Patent Publication (Kokai) No. 2003-171686 discloses a method that utilizes the interaction of the polymer with water to provide a lubricant effect by making a polymer gel having a monolayer of a hydrophilic polymer gel with a non-planar surface to control the contact area. However, such a polymer gel also does not have sufficient mechanical properties.
Methods in which water soluble polymers are used as a lubricant thereby reducing the frictional resistance in water have also been used. Such a system does not maintain its lubricity because the water soluble polymer tends to migrate away from the surface or dissolves in water.
Therefore, there remains a need for a polymer gel structure that has sufficient, maintained, lubricity while still having sufficient mechanical strength.